1. Field of the Invention
The present invention relates to a centralized data transmission method using a carrier sense multiplexing access (CSMA) method. More particularly, the present invention relates to a data transmission method in which an access point transmits data by transmitting a multi-poll message using the CSMA method.
2. Description of the Related Art
FIG. 1 illustrates a timing diagram for a conventional approach for transmitting data using a distributed coordination function (DCF) method for centralized data communications using a carrier sense multiplexing access (CSMA) method according to the prior art.
In a CSMA centralized data communications system using a wireless LAN system, such as one based on IEEE 802.11, in order to minimize contention, each station generates a random backoff number. If the channel is idle longer than a predetermined time (DCF Inter Frame Space, DIFS), the station successively decreases the backoff number, until it reaches zero (0), at which time the station transmits data. This is referred to as a DCF method. Here, the backoff number is a randomly generated number between a minimum value (CWmin) and a maximum value (CWmax) of a contention window (CW) determined by an access point.
Referring to the example shown in FIG. 1, station 1 transmits data and the backoff numbers of station 2 and station 3 are three (3) and five (5), respectively. One DIFS after station 1 ends data transmission, the backoff number of station 2 is counted down, and data is transmitted. While data of station 2 is being transmitted, counting down of the backoff number of station 3 is stopped. One DIFS after station 2 has ended data transmission, the backoff number of station 3 is counted down, and when the backoff number reaches zero (0), data is transmitted.
Since the DCF method works on a probability basis, there is a high possibility of contention between data transmitted by stations. Additionally, since the access points do not typically control the number and size of data packets, it is difficult to support time-limited services.
FIG. 2 illustrates a timing diagram for a conventional data transmission method using a point coordination function (PCF), in which a contention free period (CFP), which can be controlled by an access point, is set, and transmission opportunities are provided to each station based on a polling method. This is referred to as a point coordination function (PCF) method. In the PCF method, a PCF Inter Frame Space (PIFS) and a Short Inter Frame Space (SIFS), which are shorter than the DIFS, are used instead of the DIFS in order to obtain priority over DCF-using stations. However, in wireless LAN systems, for example, Intersil and ATmel, the PCF function is not offered due to the complexity of implementation.
FIG. 3 illustrates a timing diagram for a conventional data transmission method of a centralized data communications system using the CSMA of FIG. 1, but wherein contention period (CP) intervals and contention free period (CFP) intervals are mixed and used together. A leading beacon message may contain information on CFP intervals or CP intervals, or beacon message intervals. One SIFS interval after one station ends data transmission, an access point transmits a polling message (D1+Poll, D2+ack+Poll, D3+ack+Poll). The polling messages are transmitted to the stations from the access point. The stations that receive the polling messages transmit data and a confirmation message to confirm the transmission of the data to the access point.
The DCF method is used during the CP interval, while the PCF method is used during the CFP interval. Since the DCF method depends on obtaining transmission opportunities based on probability, it is difficult to support time-limited services. However, use of polling stations using the PCF method during the CFP interval does allow for time-limited services, but requires that a polling message be transmitted thereby further requiring additional timing management for the CFP intervals. Thus, implementation is complicated.
In addition, since the CFP interval is typically fixed, data transmitted by a polled station may exceed the CFP interval. In this case, the station has to give up the PCF transmission, and transmit the remaining data fragment in a CP interval on a contention basis. For example, when stations that received polling messages transmit data and a confirmation message (U1+ack, U2+ack or U3+ack) to confirm the transmission of data, a station that desires to transmit U3 data cannot transmit the data in the CFP interval and has to transmit the data in the CP interval.
If a polled station is to transmit continuous fragmented data, it may be necessary to adjust the lengths of a transmission frame and a CFP interval in order to practically support time-limited services. For some services, if very short interval polling is requested, polling in the CP interval, as well as polling in the CFP interval, may be needed. To solve this, the length of a transmission frame should be shorter than a corresponding polling cycle interval.